1. Field of the Invention
This invention relates to an apparatus and method for performing a soft handoff. More particularly, the invention relates to a mobile station apparatus configured to reclassify interfering RF signals as desired RF signals for soft handoff purposes.
2. Description of Related Art
In a wireless network multipath fading and interference are difficult problems to overcome. Multipath fading occurs when a transmitted signal takes more than one path to a receiver and some of the signals arrive out of phase, resulting in a weak or fading signal. Interference is an electromagnetic disturbance that interrupts, obstructs, or otherwise degrades or limits the effective performance of communications between the mobile station and the base station.
In order to combat multipath fading and interference, receivers incorporate rake receivers and smart antenna technologies. Rake receivers have the ability to selectively demodulate multipaths and combine them coherently at the mobile station receiver. Each of the rake receivers in a mobile station receiver may be used to demodulate a signal path from the same base station or the rake receiver may be used to demodulate a signal path from a different base station, i.e. a soft handoff scenario. A smart antenna augments the rake receiver performance by selecting the optimal received signal seen at the input of the rake receiver.
For purposes of this patent, the term “smart antenna” refers to various aspects of smart antenna system technology that includes intelligent antennas, phased arrays, spatial processing, beam forming, digital beam forming, adaptive antenna systems, and related adaptive RF technologies. An illustrative example of a smart antenna system is the INTELLICELL® smart antenna solution, developed by ArrayComm, Inc. of San Jose, Calif. In particular, the INTELLICELL® smart antenna uses an antenna array coupled with sophisticated digital signal processing to manage the energy that is radiated and received by a base station.
To date smart antenna systems have focused on communicating the “best” signal to the mobile station from a serving base station. In operation, these smart antenna systems scan for an RF signal in multiple antenna configurations and then proceed to determine which configuration results in the optimal received signal quality. Generally, the smart antenna system then avoids antenna configurations that would lead to high interference levels to improve signal quality.
However, technologies that use “soft handoff” have the benefit of combining signals from two or more base station transmitters to improve receiver performance. Except for RF signals generated from a serving base station, smart antenna systems interpret signals generated from any other base station as interference. Improperly interpreting a desired RF signal, which can be used for soft handoff purposes, as interference can degrade performance because the benefits of soft handoff are completely lost.